Question

What law required slaves who had escaped to free states to be forcibly returned to their owners in the South if caught?

Answer 1

That was the Fugitive Slave Act — especially the stricter Fugitive Slave Act of 1850 (the earlier Fugitive Slave Act of 1793 also authorized recapture). The 1850 law, passed as part of the Compromise of 1850, required federal officials and citizens in free states to assist in returning escaped enslaved people, imposed penalties for helping runaways, and denied jury trials for the accused.